Building Ordering a tree as a list in C#

Question

I have a list of C# entities. My entity is defined as follows:

public class Item
{
    // the id of an item
    public Guid ID { get; set; }

    // if this          


        

Answer 1

Even though you said that you wanted the items sorted by level (ascending) and then createDate (ascending), your diagram says otherwise. It looks like you actually want the items sorted in a way that would allow you to print out a tree, such that each item is after its parent and before its children and its parents "younger siblings". This is a topological sort which is substantially different from what you asked for, but not that hard:

class ItemComparer : IComparer
{
    // allow us to look up parent Items by GUID
    IDictionary itemLookup;

    public ItemComparer(IEnumerable list)
    {
        itemLookup = list.ToDictionary(item => item.ID);
        foreach (var item in list)
            SetLevel(item);
    }

    int SetLevel(Item item)
    {
        if (item.Level == 0 && item.ParentID.HasValue)
            item.Level = 1 + itemLookup[item.ParentID.Value].Level;
        return item.Level;
    }

    public int Compare(Item x, Item y)
    {
        // see if x is a child of y
        while (x.Level > y.Level)
        {
            if (x.ParentID == y.ID)
                return 1;
            x = itemLookup[x.ParentID.Value];
        }
        // see if y is a child of x
        while (y.Level > x.Level)
        {
            if (y.ParentID == x.ID)
                return -1;
            y = itemLookup[y.ParentID.Value];
        }
        // x and y are not parent-child, so find common ancestor
        while (x.ParentID != y.ParentID)
        {
            x = itemLookup[x.ParentID.Value];
            y = itemLookup[y.ParentID.Value];
        }
        // compare createDate of children of common ancestor
        return x.CreateDate.CompareTo(y.CreateDate);
    }
}

Invoke it like this:

// if List
items.Sort(new ItemComparer(items));
// if IEnumerable
var sorted = random.OrderBy(x => x, new ItemComparer(random));